If you act on your own, you have little strength and influence on the direction of the company’s development. If you demand a rise or an improvement in working conditions and your employer refuses to do so, all you can do is to accept such a refusal.
Thanks to working within a trade union, employing collective efforts and speaking with one voice, you can do much more than by doing it yourself. Being a member of a trade union means that there is always someone on your side, someone who shares your reasons. Solidaritet is superiority of the group over entity!